This invention relates to the generation of hydrogen chloride (HCl) by the catalytic conversion of halogenated hydrocarbons ("HHCs" for brevity), and more particularly to the generation of HCl in a fluidized bed of an alumina catalyst. The conversion to HCl and dischargeable products is particularly facilitated by an alumina support ("catalyst-support") for metals having a desirable catalytic action, which metals particularly include iron and/or copper. The HCl is recovered for reuse.
The problem of disposing of HHCs has confronted many persons skilled in the art. That this invention is directed to the same subject matter is evidence that the problem is fraught with difficulty, and the elusive best solution has yet to be proferred. Part of the difficulty lies with the varied considerations which define the problem, as it presents itself in different guises, hence the elusiveness of the solution; and by no means a minor part lies in the unforgiving economics of any solution to the problem. It is axiomatic that solutions to industrial problems must be economically acceptable.
As is well known, the economics of recovering HHCs from waste streams are such that, at present, the only viable alternative is conversion of the HHCs, provided such conversion results in an acceptably clean effluent for release into the environment. Thus, optimally, the HHCs are converted to carbon dioxide, water and HCl. Typically, the HCl is scrubbed out of the effluent with water in a scrubber, the aqueous acid is neutralized with lime and discarded.
A particular, early solution to the general problem of dispossal of HHCs was addressed in U.S. Pat. No. 3,140,155 where it is taught that the HHCs may be incinerated at a temperature in the range from about 900.degree. C. to 1300.degree. C. in a furnace specially designed to minimize the hazards of explosion. Notwithstanding the merits of the furnace, such incineration of HHCs was eschewed in favor of their catalytic conversion, as for example, described in U.S. Pat. No. 3,705,010 which teaches that brominated HHCs may be catalytically converted, without the hazards of incineration (non-catalytic) over particular metal oxides such as Cr.sub.2 O.sub.3, V.sub.2 O.sub.3, Mo.sub.3 O.sub.8, WO.sub.2, CeO.sub.2, U.sub.3 O.sub.8 and the like, supported on conventional supports such as alumina, using an excess of oxygen to convert the brominated HHC.
Another teaching which favors the catalytic conversion of HHCs is found in U.S. Pat. No. 3,453,073 which discloses that water and HHCs with necessary oxygen, passed over a bed of acid-type catalyst at a temperature in the range from about 300.degree. to about 600.degree. C., produces the desired HCl when the HHCs are chlorinated. The water must be present in at least a stoichiometric amount, that is, one mole of water for each mole of halogen on a monoatomic basis. The favored catalyst is phosphomolybdic acid deposited on catalyst-supports such as active carbon, silica-alumina, chromia-alumina and the like.
It is evident that catalytic conversion of HHCs is the preferred mode for their disposal and, if desired, the HCl may be recovered and recycled, but as is true of any catalytic conversion, some catalysts are more preferred than others for a host of reasons. The process of this invention is particularly directed to such a preferred catalyst which, coincidentally, is generally disclosed in the prior art for other purposes, specifically the oxidation, ammoxidation and oxyhydrochlorination of monoolefins, as in U.S. Pat. No. 4,226,798, inter alia. This prior art catalyst, like the catalyst used in the present invention, comprises an alumina catalyst-support on which is deposited a "soft" element of Groups I, V, VI and VIII of the Periodic Table, and compounds thereof. Such one or more soft elements may be deposited conventionally from salt solutions prior to use in the reaction, or the elements may be deposited in situ during the reaction. Specific soft metals are copper, iron, bismuth, antimony and the like which additionally may be promoted by the rare earth elements and elements of Groups II, IV, and VII.
Though much desired from the standpoint of the catalytic conversion of HHCs, such a catalyst unfortunately displays the peculiar phenomenon described as "tackiness" or "stickiness", which is not necessarily viscosity as conventionally defined. Stickiness is defined as the degree of particle-to-particle agglomeration, viscosity or resistance to separation of consitituent particles. This proclivity of stickiness attributable to the catalyst is described in greater detail in the '798 reference, the disclosure of which is incorporated by reference thereto as if fully set forth herein.
As stated in the '798 reference, this stickiness is attributable to the presence of an excess of HCl. Therefore, in the oxyhdrochlorination of ethylene to produce 1,2-dichloroethane ("EDC"), it is essential that a bare minimum of excess HCl be fed to the reactor if maximum conversion of ethylene is to be obtained, both from the standpoint of effective use of the reactants, as well as from the standpoint of minimizing the corrosive effects of chlorine. Thus, despite the desirable effectiveness of a "soft metal" catalyst, it was expected that stickiness of the catalyst with an excess of HCl would rule out the use of a bed of such a catalyst fluidized with an excess of HCl. Further, it was found that catalytic oxidation of HHCs which are chlorinated ("CHCs" for brevity) while fluidized with HCl without added steam, substantially decreased the conversion of CHCs. Yet, because of heat-transfer considerations, it is essential that a fluid-bed reactor be used if the process is to be an economically viable one; and, because of the known effective conversion of HHCs by the prior art catalyst, it was highly desirable to use it in a fluid-bed reactor.
Recognizing that fluidization is a necessary condition for an economical process, and this process requires oxygen, it is logical to use air as the gaseous medium. This would normally present no difficulty in the situation where the effluent from the bed is to be vented to the atmosphere, or recycled to a process where handling the nitrogen in the air is not penalized with undue costs. However, where the effluent is desirably recycled to a process where nitrogen is a burden, another choice must be made. A choice of oxygen, without nitrogen, in an amount sufficient to fluidize the bed, not only is more of an expense than the expense of handling the nitrogen as air, but pure oxygen in excess is not only an obvious explosion hazzard, but also deleterious to the catalyst.
Another choice is predicated upon the face that water is a product of the catalytic combustion of HHCs. Though steam is the next logical choice, acting, as suggested in the '073 reference, as a diluent, experimental evidence presented herebelow indicates that injecting steam to help fluidize a reactor containing the catalyst ("fluidization steam") adversely affects the conversion of the HHCs, presumably because the active sites of the catalyst are "blinded" by the injected steam.
Still other major products resulting from the conversion of HHCs are carbon dioxide and HCl or HBr, depending upon whether the HHCs are chlorinated or brominated, and these products also offer themselves as choices. However, though the effect of HBr in a fluidized bed of supported soft metal catalyst is not known, HCl has already been taught to create an undesirable stickiness problem in a fluid bed. Further, use of HCl would be expected to produce chlorine when mixed with oxygen and contacted with catalyst at a temperature above 350.degree. C., which reaction is the basis for the erstwhile Deacon process. Such production of chlorine is highly corrosive, and is proscribed, as taught in U.S. Pat. Nos. 4,031,149 and 4,169,862. More important, free chlorine at a temperature in the range specified herein for our process, results in chlorinating EDC (and any other less chlorinated HHCs) to perchloroethylene, hexachlorobenzene, hexachloroethane and the like which, being highly chlorinated HHCs are known to defy conversion under the process conditions tolerated by the equipment.
Weighing the foregoing choices, it is apparent that any choice of a fluidization medium which is also a product of the conversion of CHCs is likely to decrease such conversion of CHCs because of the law of mass action, irrespective of the effect of the medium on the catalyst, or the cost of using that medium. Thus, the choice of HCl, CO.sub.2 or steam in the large amounts necessary to fluidize a bed, would be deemed undesirable.
We are well-acquainted with the oxyhydrochlorination of ethylene (the "oxy" process) and, recognize that in this oxy process it would be highly desirable to recycle unconverted EDC and HCl which are produced by the conversion of undesirable CHCs in another reactor, as is disclosed in U.S. Pat. Nos. 3,968,200 and 4,351,819. However, to utilize such a recycle in which HCl is a desirable component, the problem to be solved was to counter the obvious disadvantages of using HCl as the fluidization medium, or in so large an excess that it constituted a substantial if not major portion of the medium. An acceptable solution required that the effectiveness of the catalyst with respect to converting essentially all the HHCs, be maintained. The process of this invention provides such a solution.